1. Field of the Invention
The present invention relates to a polarity detection technique of a video synchronizing signal that enables screen display.
2. Description of the Related Art
Apparatuses that determine the polarity of a video synchronizing signal have conventionally been proposed. For example, Japanese Patent Laid-Open No. 8-339174 (to be referred to as literature 1 hereinafter) proposes a method of counting the pulses of a horizontal synchronizing signal and comparing a counter value during the period of each polarity of a vertical synchronizing signal, thereby determining the polarity of the vertical synchronizing signal. Japanese Patent Laid-Open No. 2-6574 (to be referred to as literature 2 hereinafter) proposes a method of inputting a synchronizing signal into an integrating circuit and exclusive-ORing the integration result and the synchronizing signal, thereby deciding the synchronizing signal. Note that the proposal of literature 2 assumes that a signal of at least one period (at least one frame) is input to the integrating circuit.
When synchronizing a video signal by the method described in literature 1 or 2, a time of one frame is necessary to determine the polarity of the synchronizing signal of the video, and a long time is needed for screen display. There has also been proposed a method of, when determining the polarity of a synchronizing signal, hard-coding one of the polarities as the polarity of the synchronizing signal. In this method, when the polarity matches the hard-coded polarity, screen display can quickly be done. However, in case of an opposite polarity, a long time is necessary for screen display.